1. Field of the Invention
The present invention relates generally to photoelectric sensors and, more particularly, to photoelectric sensors which provide a discontinuity in an outer surface of a transparent portion of the sensor to inhibit the formation of liquid droplets on the outer surface.
2. Description of the Prior Art
Photoelectric sensors are well known to those skilled in the art. Some photoelectric sensors comprise a housing structure which is shaped as a polyhedron while others are generally cylindrical. All photoelectric sensors provide some means for permitting light to pass through a portion of the housing toward a photosensitive device and some have a light transmissive member, or window, formed as an integral part of the sensor housing. U.S. patent application Ser. No. 07/662,864, which was filed on May 20, 1991 and is assigned to the assignee of the present application, describes a photoelectric device with a lens formed in its housing. However, it should be understood that many photoelectric sensors have housings made from several components that are attached together to form a housing structure and window. In addition, the window can be contained in any surface of the housing.
The formation of a light transmissive surface as an integral part of a sensor housing is one technique that has been employed to prevent moisture from leaking into the internal portion of the photoelectric device. By eliminating the normal seam between a lens and a housing structure that is necessary if the lens is not formed as an integral part of the housing structure, leakage through the seam is naturally eliminated. Many such steps have been taken in an attempt to waterproof the photoelectric sensor for use in harsh duty environments, particularly where the sensor must be periodically washed with a spray of liquid. Whether the end portion of the photoelectric sensor is merely light transmissive or formed into one or more lenses, the combination of the lens and the housing in a one piece structure improves the waterproof characteristic of the device.
When a photoelectric device is exposed to liquids, there exists the probability that the liquid will contact the smooth outer surface of its light transmissive portion, or window, and form droplets which can adhere to that smooth surface. When this occurs, a droplet can reflect light emanating from the photoelectric device back into the device to cause a false signal. This false signal can emulate the reflection of light from an object, as in a diffuse photoelectric application, or the reflection of light from a reflector, as in a retroreflective sensor application. In either case, the droplet of liquid on the face of the photoelectric sensor causes an erroneous signal and adversely affects the operation of the device. Although liquid droplets can adversely affect many different types of photoelectric sensors, this problem is severely exacerbated when the light transmissive portion of the housing structure is reduced in size to place the light source and photosensitive component close together. The possibility that a liquid droplet will be of sufficient size to adversely affect the sensor is increased when the light source and light detector are confined within a smaller space.
In view of the problem described above, it would be desirable to provide a photoelectric device that is not susceptible to false signals caused by droplets of liquid adhering to the outer surface of the light transmissive portion in a photoelectric sensor.